A portmanteau, or portmanteau word, is a blend in which parts of two or more words — or of their sounds — are fused into a single new word that carries both meanings at once: smog from smoke and fog, motel from motor and hotel.
Lewis Carroll coined this sense in Through the Looking-Glass (1871), where Humpty Dumpty glosses slithy as "lithe and slimy" and explains: "You see it's like a portmanteau — there are two meanings packed up into one word." The metaphor is luggage. English took the word from Middle French portemanteau, built from porter, "to carry," and manteau, "cloak," and it named a stiff traveling case hinged to open into two equal compartments. That original sense is still recorded first in most dictionaries; the blending sense is the one in ordinary use.
A portmanteau is not a compound. A compound joins whole words and leaves both intact — bookstore, football — while a portmanteau clips them and overlaps the fragments, as in brunch, podcast, and emoticon. It is also not an 📝eponym, which converts a name into a word rather than fusing two words. In linguistics the term carries a narrower technical meaning as well: a portmanteau morph is a single form that represents two or more morphemes simultaneously, as French au stands for both à and le. A related extension, the portmanteau film, names a feature assembled from several short stories.